I was studying models for constructing phylogenetic trees. It is mentioned that all other models are 'nested' within GTR model i.e. can be arrived upon by constraining some parameters in GTR. Why should we do a model test at all and instead use a GTR model every single time because as I understand, it includes other models within itself. There are some papers where they mention "HKY/(other nucleotide model) was found to be optimal based on AIC values".